1. Field of the Invention
The present invention relates to a drive unit and a control unit for the adjustment of the braking torque of the drive unit, as well as method for controlling the drive unit.
2. Description of the Related Art
A drive unit in generic terms is known from German patent document no. DE 37 13 580 C1.
On that particular known drive unit, the retarder serves to circulate the coolant in the coolant circuit during normal traction operation as well as during continuous braking operation (xe2x80x9cwater pump-retarderxe2x80x9d). In this method, the retarder is controlled by a suitable valve arrangement so that it may also provide a braking function if necessary. During the xe2x80x9cpumpingxe2x80x9d function, the power requirement should be as low as possible, while during the xe2x80x9cbrakingxe2x80x9d function, it should be as high as possible. The technical requirements, therefore, are very conflicting. This results in the power consumption of the xe2x80x9cpumpingxe2x80x9d function being too high for sufficiently efficient operation.
However, if these two functions are structurally separated by providing a separate pump in addition to the retarder, then the retarder and the pump can be designed such that the functions can be performed with optimal efficiency. However, the space requirement for such a system is too high. This is a disadvantage since space in vehicles is very tight, especially at the pertinent location.
A retarder known from U.S. Pat. No. 3,720,372 is integrated with the drive motor, is permanently connected with the crankshaft, and has coolant from the cooling unit continuously flowing through it. The retarder rotor functions as a circulating pump in this situation, rather than utilizing a separate coolant pump. The purpose of this equipment is to heat the coolant by use of the retarder in order to warm the passenger compartment. This purpose is also facilitated by a control unit which is provided at the retarder and which serves only to control the distribution of the coolant depending on its temperature in a bypass line through the cooler.
A retarder is further known from German patent document no. DE-PS 33 01 560 which is connected with the crankshaft of the drive motor and the vehicle""s traction wheels through a shiftable coupling. The purpose of the retarder, however, is not the pick-up and conversion of high kinetic brake energy into heat. The retarder is operated only as a heating device whereby the heating capacity is to be controlled under consideration of an available drive capacity. The motor coolant is, at the same time, the operating fluid for the retarder.
A retarder which is known from German patent document no. DE-AS 1 946 167 is connected directly with the crankshaft of a combustion engine whose coolant also serves as operating fluid for the retarder. The advantage of this operational mode is that the developed heat is generated directly in the coolant that is fed into the cooler, making a heat exchanger between two fluids dispensable.
German patent document no. EP 95113594 describes a drive unit with a motor and a hydrodynamic retarder. A pump impeller, which is arranged axially to the rotor blade wheel, is provided for transportation of the coolant.
The present invention provides a drive unit of the type described above, whereby optimum operation of the drive unit is possible under all operating conditions. Particular emphasis is on the requirement of being able to variably adjust the retarder""s torque within a large range so that a large adjustment range results for the braking torque during brake operation.
According to the method of the invention, a large control range is provided for the retarder and a predetermined braking torque is established for the retarder. The braking torque is then adjusted so that devices for controlling the flow through the feed line to and through the retarder, as well as through the bypass line past the retarder, are triggered such that an initial predetermined part of the operating medium is fed through the retarder and a second predetermined part is fed past the retarder.
In addition to the method for controlling the braking torque, the invention also provides an appropriate control unit. In advancing the invention further, provisions can be made so that the feed lines and bypass lines are part of a combined cooling and braking circuit, or merely part of a coolant circuit.
By using conventional components, the two connected, previously mentioned circuits may be produced. With the assistance of suitable devices, the two circuits can be supplied with operating medium in such a manner that they are always being operated in an optimum braking, as well as non-braking mode. In one embodiment, the device or the control element for supplying the various circuits includes a valve.
In a first design variation, the valve takes the form of a two-position selector valve, whereby operating medium in the first position is fed through the bypass line. In one embodiment, operating medium is only in the cooling circuit. In the second position, the operating medium is routed through the feed line and thereby into the combined cooling and braking circuit, providing a selective supply of one or the other circuit. In such an arrangement, control of the braking torque is only possible between the maximum retarder braking torque and a minimum retarder braking torque since water is continuously flowing through the retarder. A lower deviation of the minimum retarder braking torque of the operating medium at the established flow in the combined cooling and braking circuit is not possible in this design variation because of the external resistivity. The aforementioned valve is used to select between two operational conditions, either only cooling operation or combined cooling and braking operation, and can be arranged as a {fraction (3/2)} selection valve.
The minimum retarder braking torque (MRetmin) results at a predetermined total coolant flow volume in the combined cooling and braking circuit. In order to be able to reduce the retarder""s braking torque below MRetmin, provisions may be made to permit intermediate positions between only cooling operation and combined cooling-braking operation, in place of selective switching. This results in a first partial flow of a predetermined part of the total coolant flow which is fed through the hydrodynamic retarder. In other words, the first partial flow of coolant is run in the combined cooling-braking arrangement, and a second predetermined part of the total coolant flow is routed only in the cooling circuit arrangement.
Possible devices that may help achieve the aforementioned result can, again, be valves which permit division of the total flow. For example, in the preferred design, these devices may be selector valves with at least three selection positions. The first position serves to route the operating medium only through the cooling circuit. In the third position, the operating medium is routed only through the combined cooling-braking circuit. In the second position, which is the aforementioned center position, the cooling flow can be separated into a first partial flow to be run in the cooling circuit and a second partial flow to be run in the cooling-braking circuit. It would, of course, be possible to provide a continuously adjustable division of the total flow in the second control position, or to effect settings other than those described. In a preferred design form, a selector valve, such as the one described above, is arranged as a {fraction (3/2)} selector valve which is arranged as a valve having three connections and three positions.
In addition to arranging the valve as a selector valve, it is also possible to arrange it as a continuously regulating valve, also known as continuous valve. A continuous valve, perhaps with the assistance of a piston, permits continuous regulation of the partial flows for the cooling circuit or for the combined cooling-braking circuit. Such an arrangement distinguishes itself in that it is particularly simple.
In another embodiment of the invention, a simple branching arrangement with downstream throttles can be used in place of the valve arrangement to regulate the ratios of the partial flows in the cooling circuit as well as in the combined cooling-braking circuit by appropriate adjustment of the throttles.
Through the invention, it can further be guaranteed that the operating medium circulates substantially without losses during non-braking operation.
Through a combination of cooling and braking circulation, the hydrodynamic performance potential of the pump may be used to activate the retarder. This is possible without having to change the hydrodynamic ratios.
In contrast to the arrangement in which the feed line to the retarder is part of a combined cooling/braking circuit and the bypass line is part of the cooling circuit, in another embodiment of the invention, the feed line and bypass line are only part of an operating medium circuit. The operating medium circuit can be, for example, part of an oil circuit with a secondary retarder. In the apparatus according to the invention, or in the method according to the invention, the control range for the adjustable retarder braking torque may also be increased in such an arrangement.